If it's gold plated it can wear off and the base metal underneath can show through. Then it need to be re-plated with new gold. If the plating isn't too thick, it will of course wear off faster. But I don't suppose the jeweler ever said it would just be gold-plated and so you rightfully expected (and probably paid for) solid gold.

But gold is gold--it is a noble metal that is elemental. It doesn't react with most other chemicals (which is why it is so valuable) but can react with chlorine, fluorine, cyanide and some very strong acids (aqua regia) and can be dissolved in mercury. You really shouldn't wear chlorine as a perfume as it really isn't a fragrance that most other people enjoy and it will make you smell like you live in a swimming pool (except in our compound where maintenance doesn't know what chlorine is for) ;-) I wouldn't recommend strong acids or mercury for your body either.

Gold certainly doesn't fade or change colour. Gold is not a dye or chemical agent.

So yes, you're being hosed. And if they say they turned it to white gold, don't believe them. White gold is an alloy of gold mixed with another white metal (nickel, silver or palladium with the first two being the cheapest). The quality of the second metal being key to the quality of the jewelery and its lasting lustre. This would involve melting down the piece and then mixing it with the other metal before re-smithing the piece. I suspect if it is white, then it is just silver.

Isn't there a government agency that regulates the jewelery market for cases like this?